The invention pertains to a rotatable cutting bit assembly, including the cutting bit and the chuck therefor, wherein the cutting bit is useful for cutting through various earth strata. Specifically, the invention pertains to a wet roof drill bit assembly, which includes a roof drill bit and a chuck which connects the roof drill bit to the drill steel, for drilling bore holes in an underground mine.
The expansion of an underground mine, such as for example, a coal mine, requires digging a tunnel. Initially, this tunnel has an unsupported roof. In order to support and stabilize the roof in an established area of an underground tunnel, bore holes are drilled in the roof. The apparatus used to drill these holes comprises a drill with a long shaft, i.e., drill steel, attached to the drill. A roof drill bit is detachably mounted, either directly or through the use of a chuck, to the drill steel at the distal end thereof. The roof drill bit is then pressed against the roof, and drilling apparatus operated so as to drill a bore hole in the roof. The bore holes extend between two feet to greater than twenty feet into the roof. These bore holes are filled with resin and roof bolts are affixed within the bore holes. A roof support, such as roof panels, is then attached to the roof bolts.
In the past, there have been several styles of roof drill bits put to use to drill roof bolt bore holes. These styles include for wet drilling, i.e., where a coolant impinges upon the cutting inserts and in the area of drilling, a roof drill bit with a male shank and a roof drill bit which uses the body forging like that for a vacuum center roof bit, but has no side wall openings and presents fluid passages at the forward end thereof as well as an opening at the rear thereof.
Referring to the male shank type of roof drill bit, U.S. Pat. No. 4,190,125 to Emmerich et al., U.S. Pat. No. 4,819,748 to Truscott, and U.S. Pat. No. 5,180,022 to Brady illustrate various embodiments of a roof drill bit with a male shank. While the specific structure may vary, the male shank style of roof drill bit typically comprises a forward head portion which carries at least one cutting insert and a shank which depends rearwardly from the head portion. A drill steel has an opening at the forward end thereof that typically receives therein the male shank of the roof drill bit. A fastener then connects the roof drill bit to the drill steel at the male shank. In the case of the embodiments of FIGS. 1 and 6 of the Emmerich et al. patent, a retention pin and a spring retainer, respectively, connect the roof drill bit to the drill steel. In the case of the Truscott patent, a retention pin assembly appears to connect the roof drill bit to the drill steel. In view of the configuration of the male shank of the roof drill bit in the Brady patent, one or more retention pins appear to affix the roof drill bit to the drill steel.
By using a pin or other retention assembly that requires a hole be through the wall of the drill steel, there is communication between the cavity of the roof drill bit which contains coolant under pressure and the outside of the roof drill bit. During operation, this communication path provides for leakage of coolant in the form of a high pressure stream.
Another style of roof drill bit uses a bit body made from a forging like that for the center vacuum roof bit, except that the side openings are absent and fluid passages are in the forward end adjacent the cutting inserts. This style of roof bit also has a central cavity which has a rear opening which receives the forward end of a vacuum center style of chuck. A retention pin passes through the side of the roof bit body an into a transverse hole in the chuck. The chuck connects the drill bit to a hollow drill steel. The fluid passages are in communication with a source of pressurized coolant through the cavity, the longitudinal bore in the chuck and the bore of the drill steel. During operation, coolant escapes from the roof bit through the connection between the chuck and the roof bit.
In the past, a center vacuum roof drill bit has been used to drill roof bore holes, and U.S. Pat. No. 4,603,751 to Erickson depicts this style of roof drill bit. In this style of roof drill bit, there is a central cavity which opens at the rear of the bit as well as openings in the side wall of the drill bit which communicate with the cavity. The central cavity receives a chuck through the rear opening. The chuck connects the drill bit to a hollow drill steel. The openings and cavity communicate via a bore in the chuck and the drill steel with a source of vacuum which, in operation, causes the particles and other drilling debris to be sucked into the cavity through the openings and then out of the drilling area through the central bore of the drill steel.
In a roof bolt bore hole drilling operation, it is important that the cutting inserts of the roof drill bit receive sufficient coolant, which is typically water, to maintain a sufficiently low temperature. Because drilling generates great amounts of heat, it is necessary to cool the drill bit to avoid, or at least to reduce, the thermal degradation of the cutting insert material. This is true for most cutting insert materials including without limitation polycrystalline diamond composite and cemented tungsten carbide-cobalt materials. It is thus important in a wet drilling operation for a roof drill bit assembly to deliver sufficient coolant to the cutting insert in an efficient fashion.
In both the male shank style of roof drill bit and the modified center vacuum style of roof drill bit using a vacuum center chuck, the connection between the chuck and the roof bit provides for communication between the pressurized coolant and the outside of the roof bit. In other words, during the drilling operation, coolant escapes through this connections. Because the roof drill bit rotates at a high rate of revolution and the coolant is under pressure, the coolant that escapes typically does so in a high pressure stream so as to spray the operator with coolant. This makes the operator uncomfortable and makes for an unpleasant working environment. This also reduces the volume of coolant which the assembly delivers to the cutting inserts thereby reducing the efficiency of the roof drill bit assembly.
During the drilling operation, the operator can encounter a mud seam in the earth strata. Because of the great amount of mud, the passages which deliver the coolant directly to the cutting inserts can plug with the mud. When this occurs, the drilling operation must cease to allow the operator to clean out the passages. The fact that coolant leaks out of this assembly results in less volume of pressurized coolant entering the passages. This means that there is less coolant entering the passages to resist the plugging thereof. Furthermore, because the coolant follows the path of the least resistance, there is a greater tendency for the coolant to flow through the connection between the roof bit and the drill steel or chuck than through the fluid passages thereby ensuring that the passage will remain plugged with mud until manually unplugged by the operator.
Until now, there has not been a roof drill bit suitable for wet drilling which uses a body like that for a center vacuum roof bit. To provide a roof drill bit for wet drilling that uses a body forging like the body forging for the center vacuum bit would permit the manufacture of the same forging of the bit body for drilling applications using the center vacuum drill bit and the wet roof drill bit. This provides for manufacturing efficiencies not heretofore available.